BASELINE GEOMORPHIC ASSESSMENT OF TOLAY CREEK SONOMA COUNTY, CALIFORNIA A REPORT PREPARED FOR SONOMA LAND TRUST Prepared by: Joan Florsheim, Geology Department University of California, Davis JUNE 2009 Florsheim 1 Tolay Creek Geomorphic Assessment Baseline Geomorphic Assessment of Tolay Creek INTRODUCTION This baseline geomorphic assessment of Tolay Creek Watershed is intended to provide information about geomorphology that may be used to aid Sonoma Land Trust to address active erosion and sedimentation processes. This information is treated within the context of natural processes, land use changes, and potential future restoration activities. In particular, this report addresses an important question: how do physical watershed characteristics and land uses govern geomorphic processes such as erosion and sedimentation in the Tolay Creek Watershed? This question is important to address due to concern that active gully and fluvial incision and bank erosion in the middle reaches of Tolay Creek and channel aggradation in downstream reaches occurred within the past decade. This work is based on a reconnaissance level investigation including field observations, map and photograph analysis, and discussions with agency staff and local residents knowledgeable about Tolay Creek. The data and results of the reconnaissance reported here is intended to provide a basis for an approach to restoration and management that accommodates geomorphic processes. Results also serve as a basis for a more detailed field work plan to initiate baseline monitoring that would be conducted during a future phase of this investigation. METHODS Methods utilized in this work included field reconnaissance, photographic analysis, historic, topographic, and geologic map analysis, review of available hydrologic, geologic, and geomorphic literature, and discussion with agency staff and residents knowledgeable about the watershed. 1. Field reconnaissance of Tolay Creek was conducted during October and November 2008 in coordination with the Sonoma Land Trust and West Coast Watershed (lower watershed), and the Sonoma County Regional Parks Department (upper watershed). The reconnaissance included walking the entire channel from Tolay Lake to Arnold Drive. 2. Topographic map analysis included delineation of the Tolay watershed boundary, measurement of the watershed drainage basin area, illustration of the drainage network, and calculation of channel gradient. Geology is well defined by recent detailed California Department of Mines and Geology mapping in the Sears Point Florsheim 2 Tolay Creek Geomorphic Assessment and Petaluma River Quadrangles and in a recent MS thesis from San Jose State University. 3. A set of five photographic images taken in 1942, 1961, 1981, 1997, and 2007 was assembled to document changes in channel pattern. Channel pattern was mapped during each time period where visible; in some reaches the imagery did not have high enough resolution to differentiate the channel from the surrounding landscape, an in other reaches riparian vegetation obscured the channel. Results are qualitative, as photographs were not ortho-rectified. 4. Tributary erosion processes were characterized using high resolution 2008 LiDAR images along the Rogers Creek Fault zone in the upper portion of the watershed. In the lower portion of the watershed, erosion processes are characterized using the 2007 SLT photograph. GEOMORPHIC AND HYDROLOGIC CHARACTERISTICS OF THE TOLAY CREEK WATERSHED Topography and Tributary Drainage Network Tolay Creek drains the Northern California Coast Ranges in Sonoma County, California (Figure 1). The Creek discharges into a tidally influenced marsh complex at the southern end of Sonoma Valley before entering San Pablo Bay. For the purpose of this report, the watershed outlet is defined as the point where Arnold Drive (Highway 121) crosses Tolay Creek. Downstream of Arnold Drive, in the tidally influenced areas of Tolay Creek, restoration research and monitoring is currently underway by USGS Western Ecological Research Center and others (Takekawa et al., 2002). Tolay Creek flows from northwest to southeast in a valley situated between and roughly parallel to both Sonoma Creek and the Petaluma River drainages (Figure 1). The watershed boundary is defined by rounded ridges and hilltops with maximum relief of about 900 feet. The highest elevation in the watershed is an unnamed peak on the ridge to the northeast of the creek at 916 ft and the elevation of Tolay Creek at Arnold Drive is about 15 ft. The watershed divide in the headwaters has a relatively low elevation and in some place is somewhat indistinct from the adjacent Petaluma River watershed—with a driveway off of Stage Gulch Road currently acting as one point along the drainage divide (Steve Ehret, Sonoma County Regional Parks Department, Personal Communication, 2008). The Tolay Creek watershed upstream of Arnold Drive has a drainage area of 8.3 mi2. The drainage network is composed of all of the tributaries and channels within the drainage basin (Figure 2). Based on 1:24,000 scale USGS topographic maps of the area, Tolay Creek is a third-order channel with a total main channel length of about 6.3 mi. Channel gradient illustrated in Figure 3 shows three zones with distinct slope, here called upper, middle, and lower. The upper basin is wide and includes both Tolay Lake and a Florsheim 3 Tolay Creek Geomorphic Assessment reach of Tolay Creek downstream of the Lake. This ~3.0 mile reach has a relatively low slope of 0.0013. The middle basin is narrow and includes a shorter ~1.2 mile reach of Tolay Creek with a steeper slope of 0.0212. The lower basin is relatively wider than the middle basin and includes a ~1.7 mile reach of Tolay Creek with a slope of 0.0069. Before the 1860's a natural dam impounded Tolay Lake in the upper portion of the watershed. Prior to its destruction, the natural dam surface was about 14 feet higher than the former lake bed (see Figure 9 in Kamman Associates, 2003). Following the initial destruction of the natural dam, the farmers rebuilt a small dam that allowed the farmers to keep water in the lake bottom for longer periods. By keeping the lake bed area moist, they were able to raise potatoes, corn, pumpkins, and squash without irrigation. Planting took place first around the outside margin of the lake, and as the lake dried, the farmers worked toward the middle of the lake. The channel through middle of lake would usually dry by the end of the summer, although wet areas could still be seen. The new small dam created the Tolay Lake of today (Marvin Cardoza, Personal Communication, 2009). There are 13 tributaries in the Tolay Creek watershed that join the main channel; the spacing of tributary junctions with the main channel is illustrated on Figure 3. Within the middle portion of Tolay Creek, tributary junctions are sites of sediment deposition and woody debris accumulation. The physical complexity at such sites in turn creates important loci of ecological diversity (Benda, et al., 2004). In the upper watershed, tributaries on the northeastern side of the Tolay Creek watershed are longer than those on the southeastern side due to tectonically imposed asymmetry of the watershed. Thus, higher ridges and longer hillslopes exist on the northeast side than on the southwest side of the creek. Other influences of tectonics on the drainage network are described in a subsequent section. Hydrologic Processes Sonoma County is characterized by a Mediterranean climate with seasonal precipitation, and storms, erosion, and sediment transport processes are highly episodic. Hydrologic processes in the watershed integrated the upstream tributaries and Tolay Lake downstream to Tolay Creek during wet periods when the lake level was high enough to spill over or around the original dam. Water would have been contributed to the downstream fluvial system as the enlarged lake drained to the elevation of the natural outfall. The extent to which groundwater processes continued to provide base flow to downstream areas after dam overflow stopped is unknown. Presumably, after removal of the natural dam, flow in the downstream portions of Tolay Creek became intermittent or at least drier for longer periods each year. Land use practices as of 2003 included draining water from the Tolay Lake basin using high capacity pumps, because of limited outlet capacity during periods when storm flow to the lake bottom exceeded the ability of the current outlet to convey flow to the downstream portion of Tolay Creek (Kamman Associates, 2003). There are no precipitation data from within the Tolay Creek Watershed, however there are several records from nearby stations. The wet season occurs between October and Florsheim 4 Tolay Creek Geomorphic Assessment May, and the dry season occurs between June and September. At Petaluma Fire Station 3 (046826), average annual precipitation between 1893 and 2007 was about 25 inches, whereas, a shorter record from Mare Island (045333) between 1961 and 1975 reports a somewhat lower average annual precipitation of about 20 inches (Desert Research Institute Regional Climate Summaries; http://www.wrcc.dri.edu/Climsum.html). A long record from the City of Sonoma between 1899 to 1907 and 1931 to 1997 reports an annual average of 29.2 inches (see Figure 4 in McKee et al., 2000). Thus, the average annual precipitation in the Tolay watershed is probably about 20-30 inches. There are no stream flow gages in the watershed; however, early rainfall history (www.calarchives4u.com/history/sonoma/sect2.htm) suggests that overbank floods occurred in Sonoma County when annual rainfall measured on the order of about 40 inches. Thus, floods in the Tolay basin may have occurred during water years 1841, 1850, 1853, 1862, 1867, 1868, and 1872—or in about seven out of 35 years. During the 82 year period between 1914 and 1996 (see Figure 6 in Kamman Associates, 2003), years with rainfall greater than 40 inches occurred only four times (during 1913, 1940, 1982, 1994), and floods are also documented in Sonoma County during 1955, 1986, and 1995.